Control of carbon nanotube diameter using CVD or PECVD growth

ABSTRACT

The diameter of carbon nanotubes grown by chemical vapor deposition is controlled independent of the catalyst size by controlling the residence time of reactive gases in the reactor.

TECHNICAL FIELD

The present invention relates to carbon nanotube growth using chemicalvapor deposition (CVD) or plasma enhanced chemical vapor deposition(PECVD), and more specifically to the control of the carbon nanotubediameter during the CVD or PECVD growth.

BACKGROUND ART

Carbon nanotube based field effect transistors (CNTFETs) show greatpromise for device applications. Recently CNTFETs with excellentelectrical characteristics comparable to state-of-the-art siliconMOSFETs have been demonstrated [see, for example, Rosenblatt et al.,“High Performance Electrolyte-Gates Carbon Nanotube Transistors”,NanoLetters, 2(8), (2002) pp. 869-72]. The electrical characteristics ofCNTFETs however depends largely on the band-gap of the single wallcarbon nanotube (SWNT) forming the channel of the transistor. Since theband-gap of SWNTs has a strong dependence on the diameter, accuratecontrol of the diameter is essential to the success of any devicetechnology based on carbon nanotubes.

A widely used technique for the growth of SWNTs with a narrow diameterdistribution is based on the laser ablation of a graphite target.However controlled placement and orientation of tubes produced by laserablation has proved extremely challenging, and limits the usefulness ofthis technique for circuit applications. A more attractive approach forcircuit integration is to grow the carbon nanotubes in place on asuitable substrate using chemical vapor deposition (CVD). Variousstudies have shown the feasibility of controlling the orientation andorigin of CVD grown carbon nanotubes using substrates with patternedcatalyst [see, for example, Huang et al., “Growth of Millimeter-Long andHorizontally Aligned Single-Walled Carbon Nanotubes on Flat Substrates”,J. American Chemical Soc., 125(2003)pp.5636-5637].

A crucial difficulty in obtaining individual SWNTs by CVD is control ofnanometric catalyst particle size at growth temperatures of 700-1000° C.It has been theorized that the particle size of the growth catalyst usedcan define the diameter of as grown carbon nanotubes. This hypothesishas been supported by the observation that catalytic particles at theends of CVD grown SWNT have sizes commensurate with the nanotubediameters [see, for example, Li et al., “Growth of Single-Walled CarbonNanotubes from Discrete Catalytic Nanoparticles of Various Sizes”, J.Physical Chemistry, 105 (2001), pp. 11424-11431]. Catalysts typicallyemployed are transition metals, notably Fe, Mo, Co, NI, Ti, Cr, Ru, W,Mn, Re, Rh, Pd, V or alloys thereof. However, the synthesis of smallcatalyst particles with a narrow diameter distribution is complicatedand difficult to control.

Numerous strategies have been employed to control catalyst size and thusCNT diameters. One of the difficulties is to prevent catalystagglomeration during growth of the CNT at elevated temperatures. Onestrategy has been to embed the catalyst particles in a high surface areasilica or aluminosilicate matrix, which does not sinter during heatingas detailed by Cassell et al., J. Am. Chem. Soc. 1999, 121, 7975-7976.Although a high yield of SWNT is obtained with the catalyst matrix, thematrix is extremely rough and nonconductive making patterning andelectrical contact difficult. Other approaches to control the catalystparticle size include the preparation of dilute discrete nano particlesin solution utilizing ferrite (iron storage protein) micelles, polymersor other coordinating reagents that bind the surface of thenanoparticles and prevent them from growing bigger, as described by Liet al., supra. However, the synthesis of dilute discrete nano particlesin solution suffers from the difficulty of tightly controlling particlesize and limiting their agglomeration during CNT growth. Additionally,the dilute nature of the mixtures typically results in a lower SWNTyield, presumably due to the lower density of active sites. Otherdifficulties include the need to remove excess organic materialsurrounding the metal nanoparticle by pyrolysis and to reduce the oxideto activate the catalyst.

Another drawback of controlling the carbon nanotube diameter by catalystparticle size is the inability to define sufficiently small catalystparticles by lithography. Sub 1 nanometer dimensions are beyond therealm of even e-beam lithography and preclude the possibility oflithography patterning individual catalyst particles for growth of CNTswith small diameter. Thus growth of thin nanotubes (<5 nanometers) fromlarge catalyst particles (>20 nanometers) which can be patternedlithographically has numerous advantages from an integration standpoint.

One of the main challenges facing carbon nanotube based electronics isthe low drive currents of present-day CNTFETs. The low drive currentstems from the extremely small diameter of SWNTs effectively resultingin a transitor with a narrow width. Using arrays of SWNTs for thechannel region will increase the drive current, making CNTFET basedtechnologies feasible. However, at the present time no controlled waysexist forming arrays of CNTs with a well defined pitch. Thus the abilityto grow arrays of SWNTs with lithographically controlled origins(limited by ebeam resolution) and small diameters (<5 nanometers) iscrucial to the success of CNT electronics.

SUMMARY OF INVENTION

The present invention provides a method for controlling the diameter ofSWNTs using CVD or PECVD growth conditions.

The present invention relates to controlling the diameter, d_(cnt) ofCVD or PECVD grown CNTs based on the control of the residence time ofthe gases in the reactor such as by controlling the pressure, or the gasflow rates, or a combination of both, independent of catalyst particlesize. As defined by Grill in “Cold Plasma in Materials Fabrication FromFundamentals to Applications” published by IEEE press, 1994, page 91,the gas residence time is: t_(r)=p vol_(r)/Q

Wherein

-   p=pressure (atmospheres)-   t_(r)=volume of reactor (ccm³)-   Q=total mass flow (sccm)

The gas residence time is a measure of the average time of the gas inthe reactor. Thus, if the flow is constant and the pressure increases,the residence time increases, and if the pressure is constant and theflow increases the residence time decreases. The present inventors haveunexpectedly discovered that by varying the residence they can influencethe d_(cnt). If the residence time is too high, only pyrolytic carbon isdeposited and if the residence time is too low, nothing is deposited.The residence time is typically about 1 minute to about 20 minutes andmore typically about 1 to about 10 minutes. The residence time istypically determined by controlling the pressure, flow or both thepressure and flow in the reactor. By varying the residence time (e.ggrowth pressure and/or flow rates) of the CNT precursor gases in the CVDor PECVD reactor, d_(cnt), can be varied from about 0.2 nanometers toseveral nanometers.

In particular, the present invention relates to a method for controllingthe diameter of carbon nanotubes grown by chemical vapor deposition(CVD) or plasma enhanced (PECVD) in the range of about 0.2 to about 100nanometers comprising:

introducing a catalyst coated substrate into a CVD or PECVD growthreactor;

increasing the reactor chamber temperature to a desired growthtemperature;

flowing reactive gases including a carbon containing precursor over thecatalyst; and

controlling the residence time of the reactive gases in the reactor forcontrolling the diameter of the carbon nanotubes.

According to preferred aspects, the present invention relates to amethod for controlling the diameter of carbon nanotubes grown bychemical vapor deposition (CVD) or plasma enhanced (PECVD) in the rangeof about 0.2 to about 100 nanometers comprising:

introducing a catalyst coated substrate into a CVD growth reactor;

increasing the reactor chamber temperature to a desired growthtemperature;

flowing reactive gases including a carbon containing precursor over thecatalyst;

establishing a controlled pressure or controlled gas flow rate or bothin the reactor;

adjusting the gas flow of the carbon containing precursor in the case ofa controlled gas flow for controlling the diameter of the carbonnanotubes; or adjusting gas pressure in the reactor in the case of acontrolled pressure to control the diameter of the carbon nantubes; orboth adjusting the gas flow of the carbon containing precursor; and

adjusting the pressure in the reactor in the case of both a controlledgas flow of the carbon containing precursor and controlled pressure inthe reactor; to control the diameter of the carbon nanotubes.

A further aspect of the present invention relates to fabricating a SWNTor array of SWNTs having well defined diameters and origins by the abovedisclosed processes wherein the SWNTs form the channel of a field effecttransistor.

A still further aspect of the present invention is a CVD or PECVD growncarbon nanotube obtained by the above disclosed process.

Another aspect of the present invention relates to a SWNT or array ofSWNTs forming the channel of a field effect transistor obtained by theabove disclosed process.

Another aspect of the present invention comprises a field-effecttransistor having source and drain regions and a channel located betweenthe source and drain regions obtained by a process comprising:

depositing a thin film of catalyst;

lithographically patterning the thin film of catalyst to providecatalyst only in the source or drain region or both;

removing unwanted catalyst from the channel region defined by thelithographic pattern;

growing nanotube with a well controlled diameter ranging from about 0.2nanometers to about 100 nanometers by controlling the residence time ofgases in the reactor used for the growing of the nanotube and whereinthe channel region extends from the source region to the drain region.

Other objects and advantages of the present invention will becomereadily apparent by those skilled in the art from the following detaileddescription, wherein it is shown and described in the preferredembodiments of the invention, simply by way of illustration of the bestmode contemplated of carrying out the invention. As will be realized theinvention is capable of other and different embodiments, and its severaldetails are capable of modifications in various obvious respects,without departing form the invention. Accordingly, the description is tobe regarded as illustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B show scanning electron microscope images of CNTs grown atatmospheric pressure using identical catalysts, but different gas flows.(FIG. 1A shows that higher gas flow results in relatively thin tubes,while FIG. 1B shows that lower gas flows in result in relatively thicktubes).

FIGS. 2A-2B illustrate the effects of growth pressure on CNT diameter.(FIG. 2A shows a scanning electron micrograph of CNTs grown at 80 torr,while FIG. 2B shows an atomic force microscopy image of CNTs grown at 40torr using identical catalysts (2 mm thick Fe film) and gas flows.

FIG. 3 is a schematic representation of the cross-section of thestarting substrate.

FIG. 4 is a schematic representation of the cross-section of thestarting substrate with a thin catalyst film.

FIGS. 5A-5B show the cross-sectional view (FIG. 5A) and plan view of thesubstrate with patterned catalyst islands (FIG. 5B).

FIG. 6 shows a plan view of the substrate with patterned catalystislands after growth of carbon nanotubes.

BEST AND VARIOUS NEEDS FOR CARRYING OUT INVENTION

The present invention makes it possible to control the diameter of SWNTsusing CVD or PECVD growth conditions. The present invention makes itpossible to fabricate SWNTs with diameters smaller than the size of thecatalyst particles.

The present inventors have discovered that the residence time t_(r) ofthe carbon containing precursor in the reactor chamber has a stronginfluence on d_(CNT). Shorter residence times result in tubes withsmaller diameters, and d_(CNT) can be tuned by adjusting t_(r). Sincet_(r) can be controlled by adjusting the total flow rate, growthpressure or both, this method greatly simplifies catalyst preparationand control of carbon nanotube diameter.

Controlling the carbon nanotube diameter according to the presentinvention such as by growth pressure and/or flow rate relaxes theconstraints on the catalyst particle size. It is a significant advantageto be able to control the diameter using residence time such as by flowrates and growth pressure since it is much easier to control theseparameters compared to the catalyst particle size. Thus this inventionconsiderably simplifies catalyst preparation and ease of carbon nanotubeintegration for circuit applications. Catalyst systems that have beendeemed unsuitable for SWNT growth due (a) poor control of the catalystparticle size and (b) large catalyst particle size can now be utilized.

Suitable catalysts include the group of transition metals including Fe,Mo, Co, Ni, Ti, Cr, Ru, Mn, Re, Rh, Pd, V or alloys thereof. Sinceaccurate size control of the catalyst particles is not required, avariety of catalyst systems deposited by a variety of solutiondeposition, or physical vapor deposition can be utilized for CNT growth.For example, catalysts may be deposited in the form of (a) thin metalfilms (on the order of 1-2 nanometers thick) which agglomerate at thegrowth temperature forming nanoparticles of the catalyst which aretypically in the range of about 0.5 to about 30 nanometers in size (b)lithographically defined or (c) embedded into a suitable supportstructure such as porous alumina or silica. Any other suitable techniquefor achieving the desired level of size control may be employed.

The catalyst is then ramped up to the desired growth temperature in asuitable ambient prior to initiating carbon nanotube growth using acarbon containing precursor. The growth temperature is typically about400 to about 1200° C. and more typically about 500 to about 1000° C.

Suitable carbon containing precursors include but are not limited toaliphatic hydrocarbons, aromatic hydrocarbons, carbonyls, halogenatedhydrocarbons, silyated hydrocarbons, alcohols, ethers, aldehydes,ketones, acids, phenols, esters, amines, alkylnitrile, thioethers,cyanates, nitroalkyl, alkylnitrate, and/or mixtures of one or more ofthe above, and more typically methane, ethane, propane, butane,ethylene, acetylene, carbon monoxide, benzene and methylsilane. Otherreactive gases such as hydrogen and ammonia, which play an importantrole in CNT growth, may also be introduced. Also, carrier gases such asargon, nitrogen and helium can be introduced.

The residence time of the carbon containing precursor can be controlledby flow rate and/or growth pressure. By adjusting the residence timesuch as by adjusting gas flow rates and/or the growth pressure, thediameter of the carbon nanotube can be controlled.

The residence time is typically about 1 minute to about 20 minutes andmore typically about 1.2 minutes to about 10 minutes. The gas flow rateis typically about 1 sccm to about 10₄ sccm and more typically about 20sccm to about 2000 sccm. The growth pressure is typically about 1 mTorrto about 760 Torr, and more typically about 1 Torr to about 760 Torr.

The diameter of the carbon nanotubes is typically about 0.5-100nanometers, more typically about 0.8 to about 50 nanometers and evenmore typically about 1 to about 5 nanometers. The size distribution ofthe diameter is typically about 0.3 to about 5 nanometers and moretypically about 0.5 to about 1.5 nanometers.

Films according to the present invention can be deposited through a maskor patterned using conventional lithography techniques allowing thegrowth of CNTs with small diameters (<5 nanometers) fromlithographically defined catalyst islands (>20 nanometers in diameter).Thus using this technique one can easily grow an array of tubes in acontrolled manner for FET applications where the nanotubes form thechannel. Additionally, this method can be used to further narrow thedistribution in d_(CNT) for a catalyst system with a given distributionof catalyst particle size.

The following non-limiting examples are presented to further illustratethe present invention.

EXAMPLE 1

This example demonstrates the effect of flow rate on nanotube diameter.

A catalyst comprising an alumina silicate matrix impregnated with Fe/Monanoparticles is heated up to 900° C. in a hydrogen gas ambient. At 900°C., methane diluted in argon (Ar) is flowed through the reactor chamberover the catalyst. FIG. 1( a) shows a scanning electron micrograph ofcarbon nanotubes grown at atmospheric pressure for a methane flow rateof 900 sccm and no argon flow (corresponds to a residence time, t_(r),of −6.5 min). These growth conditions resulted in relatively thickcarbon nanotubes with diameters on the order of about 30 nanometers.FIG. 1( b) shows a scanning electron micrograph of carbon nanotubesgrown for a methane flow rate of 900 sccm and an argon flow rate of 1000sccm (corresponds to t_(r)-3 min). Increasing the total flow rateresulted in a lower residence time for the methane and thinner tubeswith d_(CNT)<10 nanometers as determined by scanning electronmicroscopy. Increasing the co-flow of Ar to 5000 sccm (t_(r)-1 min)resulted in no CNT growth. Decreasing CNT diameter with increasing flowrates illustrates that d_(CNT) can be adjusted by controlling the flowrate.

EXAMPLE 2

This example demonstrates the effects of growth pressure on CNTdiameter. A catalyst comprising a 2 nanometers thick patterned film ofFe is heated up to the growth temperature of 950° C. in a hydrogen gasambient. FIGS. 2( a)-2(b) illustrate the effects of growth pressure onCNT diameter. FIG. 2( a) shows a scanning electron micrograph of CNTsgrown at 80 torr and methane flow of 100 sccm (t_(r)-6 min), while FIG.2( b) shows an atomic force microscopy image of CNTs grown at 40 torrfor the same methane flow rate (t_(r)-3 min). It is evident that thelower growth pressure results in CNTs with a much smaller diameter.While the d_(CNT)=2.5±1.5 nanometers for the growth at 40 torr, the CNTdiameter is much larger (on the order of 50 nanometers) for a growthpressure of 80 torr.

From the above it is clear that the residence time can be used tocontrol tube diameter from <1 nanometer diameters up-to 50 nanometers orgreater. It is also possible to produce tubes having a narrowdistribution (<1.5 nanometers) using this technique as shown in FIG. 2(b).

The carbon nanotubes according to the present invention can be used infully integrated structures using a thin metal film. FIG. 3 shows thecross-section of a suitable substrate, 10 which for example can besilicon, silicon oxide, germanium, alloys of silicon and germanium,silicon carbide, III-V semiconductors, silicon nitride, quartz,sapphire, beryllium oxide, and aluminum nitride, other semiconductors,other insulators, conductors, such as metals or nitrides of metals. Athin film (0.5-30 nanometers) of metal catalyst such as a transitiongroup metal including Fe, Mo, Co, Ni, Ti, Cr, Ru, W, Mn, Re, Rh, Pd, Vor alloys thereof is then deposited on the substrate using a suitabletechnique such as chemical vapor deposition, atomic layer deposition,chemical solution deposition, physical vapor deposition, molecular beanepitaxy, metal evaporation, sputtering or electroplating. The catalystmay be prepared inside patterned porous structures.

FIG. 4 shows a cross-section of the thin metal film 20, deposited on thesubstrate. The thin metal film is then patterned using any lithographictechnique e.g. ebeam, optical dip-pen, micro-imprint etc. The unwantedcatalyst can be removed by either a lift-off process or by etching sothat the dimensions of the catalyst island are typically about 100nanometers or less.

FIGS. (5(a) and (b) show the cross-sectional and plan view of thepatterned substrates using the method described above. Each catalystisland acts as the nucleation center for a CNT. The CNTs can be orientedduring growth using electric fields or flow direction as has beendescribed in the literature [see, for example, Joselevich, “VectorialGrowth of Metallic and Semiconducting Single-Wall Carbon Nanotubes”,NanoLetters, 2 (2002) pp.1137-1141].

FIG. 6 shows the plan view of the final structure 50 after growth ofaligned CNTs, 40. The structure, thus comprises an array of CNTs withlithographically defined origins and well controlled diameters. Thearray of CNTs thus formed can be used as the channel of a CNT based FETfor high drive current.

The present disclosure describes and illustrates a method forcontrolling the diameter of carbon nanotubes by controlling theresidence time in a CVD reactor such as through gas flow and/orpressure, independent of catalyst particle size. This process offers asignificant advantage in terms of catalyst preparation and the growth.When used in conjunction with a catalyst system with a narrow catalystparticle size, carbon nanotubes with a very narrow diameter distributioncan be obtained. Also described above is a method for forming a novelstructure comprising an array of CNTs with well defined diameters andlithographically defined origins. This structure is suitable for formingthe channel region of CNT based FETs.

While the present invention has been described in an illustrativemanner, it should be understood that the terminology used is intended tobe in a nature of words of description rather than of limitation.Furthermore, while the present invention has been described in terms ofseveral preferred embodiments, it is to be appreciated that thoseskilled in the art will readily apply these teachings to other possiblevariations of the inventions. Also, it is intended that the appendedclaims be construed to include alternative embodiments.

All publications and patent applications cited in this specification areherein incorporated by reference as if each individual publication orpatent application were specifically and individually indicated to beincorporated by reference.

1. A method for controlling the diameter of carbon nanotubes grown bychemical vapor deposition (CVD) or by plasma enhanced chemical vapordeposition (PECVD) in the range of about 0.2 to about 100 nanometerscomprising: introducing a catalyst substrate into a CVD OR PECVD growthreactor; increasing the growth reactor temperature to a desired growthtemperature; flowing reactive gases including a carbon containingprecursor; and controlling the residence time of the carbon containingprecursor in the reactor to control the diameter of the carbonnanotubes.
 2. The method of claim 1 wherein the residence time of thecarbon containing precursor in the reactor is controlled by establishinga controlled pressure in the reaction chamber and adjusting the gas flowrate of the carbon precursor.
 3. The method of claim 1 wherein theresidence time of the carbon containing precursor in the reactor iscontrolled by establishing controlled gas flow rates into the reactorand adjusting the pressure in the reactor.
 4. The method of claim 1wherein the residence time of the carbon containing precursor in thereactor is controlled by adjusting the gas flow rate and the growthpressure of the reactor.
 5. The method according to any one of claims 1,2, 3 or 4 wherein the growth temperature is about 400 to about 1200° C.6. The method according to any one of claims 1, 2, 3 or 4 wherein thecatalyst substrate contains transition metal particles.
 7. The methodaccording to claim 6 wherein the catalyst comprises at least one memberselected from the group consisting of Fe, Mo, Co, Ni, Ti, Cr, Ru, Mn,Re, Rh, Pd, V and alloys thereof.
 8. The method according to any one ofclaims 1, 2, 3 and 4 wherein the catalyst substrate have a size about0.2 nanometers to about 100 nanometers.
 9. The method according to anyone of claims 1, 2, 3 and 4 wherein the carbon containing precursorcomprises at least one member selected from the group consisting ofaliphatic hydrocarbons, aromatic hydrocarbons, carbonyls, halogenatedhydrocarbons, silyated hydrocarbons, alcohols, ethers, aldehydes,ketones, acids, phenols, esters, amines, alkylnitrile, thioethers,cyanates, nitroalkyl, alkylnitrate, and mixtures thereof.
 10. The methodaccording to any one of claims 1, 2, 3 or 4 wherein the carboncontaining precursor comprises at least one member selected from thegroup consisting of methane, ethane, propane, butane, ethylene,acetylene, carbon monoxide and benzene.
 11. The method according to anyone of claims 1, 2, 3 or 4 which comprises employing a carrier gas alongwith the carbon precursor.
 12. The method of claim 11 wherein thecarrier gas comprises at least one member selected from the groupconsisting of argon, nitrogen, helium, hydrogen and ammonium.
 13. Themethod according to any one of claims 1, 2, 3 or 4 wherein the flow rateor pressure or both is adjusted such that the residence time in thereactor can be varied from about 1 minute to about 20 minutes, to tunethe CNT diameter.
 14. The method according to any one of a claims 1, 2,3 or 4 wherein the flow rate or pressure or both is adjusted so that theresidence time can be varied between about 1.2 minutes to about 10minutes to tune the CNT diameter.
 15. The method according to any one ofclaims 1, 2, 3 or 4 wherein the diameter of the carbon nanotubes issmaller than the particle size of the catalyst.